MDMA frequency of use

How bad is it to roll on MDMA/bk-mdma once a week for 2 or 3 weeks? I know you are supposed to wait at least a month+. Would you know which of mdma or bk-mdma is “safer”?

MDMA is much safer than bk-MDMA. MDMA has been known for over a century, there are enough data to know its effects, risks and possible problems. Research on bk-MDMA is much lesser and its risks are potentially unknown. Structural differences of these two compounds is little but, in chemistry, a simple change of an atom can mean different properties and risks.
Recommendations on MDMA spacing are general. In general 3 months is better than 1, 1 month better than 2 weeks. But is a general recommendation and not a rule.